Guard cell

Air pollutants may enter plant systems by either a primary or a secondary pathway. The primary pathway is analogous to human inhalation. Figure 8-2 shows the cross section of a leaf. Both of the outer surfaces are covered by a layer of epidermal cells, which help in moisture retention. Between the epidermal layers are the mesophyll cells—the spongy and palisade parenchyma. The leaf has a vascular bundle which carries water, minerals, and carbohydrates throughout the plant. Two important features shown in Fig. 8-2 are the openings in the epidermal layers called stomates, which are controlled by guard cells which can open and close, and air spaces in the interior of the leaf. 

Fig. 8. Activation of the PO binding with P, infestans cell walls (glucan-specific ) under pathogen inoculation and treatment with salicylic (SA) and jasmonic (JA) acids (A) Peroxidase activity in stomata guard cells and intercellular spaces of adjoining epidermal leaf cells and on the surface of mycelium contacting with the stomata (B). (1) Non-treated control (2) infection (3) treatment with SA (4) treatment with SA + infection (5) treatment with JA (6) treatment with JA + infection (7) treatment with SA + JA (8) treatment with SA + JA + infection g - gifs of P. infestans s - stomata guard cell. Specific to P, infestans cell walls, PO is highlighted.

It is of interest that the activation of POs often takes place in stomata guard cells, since P. infestans mainly penetrates into plant tissues through stomata slits. The localisation of phenolic compounds - some of them seemingly being used by POs as a substrate - and PO activity was visible in guard cells. (Maksimov et al., 2011). As such, the immune reaction occurred in close proximity to pathogen structures. 

Zeiger, E. (1983). The biology of stomatal guard cells. Annual Review of Plant Physiology, 34, 441-75. 

These gas- and water-impermeable cell layers protect the plant from desiccation, but they also hamper the uptake of carbon dioxide necessary for photosynthesis and oxygen necessary for respiration. Specialized tissues have evolved to allow passive (lenticels) and active (guard cells) modification of the permeability of the external cuticle to gas exchange. 

Guard cells Specialized epidermal cells, crescent-shaped, contain chloroplasts, form defines stomatal pore Regulate stomatal aperture/pore for gas exchange... 

Subsidiary cells Surround guard cells of stomata Reservoirs for water and ions... 

McAinsh MR, Webb AAR, Taylor JE, Hetherington AM. Stimulus-induced oscillations in guard cell cytosolic free calcium. Plant Cell 1995 7 1207-1219. 

Hwang J-U, Suh SS, Yi H, Kim J, Lee Y. Actin filaments modulate both stomatal opening and inward K+ channel activities in guard cells of Vidafaba. Plant Physiol 1997 115 335-342. 

Mirocha CJ, Schauerhamer B, Pathre SV (1974) Isolation, detection, and quantification of zearalenone in maize and barley. J Assoc Anal Chem 57 1104-1110 Muller R, Baier M, Kaiser WM (1991) Differential stimulation of PEP-carboxylation in guard cells and mesophyll cells by ammonium or fusicoccin. J Exp Bot 42 215-220 Murashige T, Skoog F (1962) A revised medium for rapid growth and bioassays with tobacco tissue cultures. Physiol Plant 15 473-497... 

Schroeder, J.L. Kwak, J.M. Allen, G.J. (2001) Guard cell abscisic acid sigialling and engineering drought hardiness in plants. Nature, 410, 327-30. 

Takahama, U., Oxidation of flavonols by hydrogen peroxide in epidermal and guard cells of Vida faba L., Plant Cell Physiol, 29, 433, 1988. 

Sophoroside-7-glucuronide Allium cepa guard cells Alliaceae 207... 

Urushibara, S., Okuno, T., and Matsumoto, T., New flavonol glycosides, major determinants inducing the green fluorescence in the guard cells of Allium cepa. Tetrahedron Lett., 33,1213,1992. 

Epidermal tissue of plants consists of flat cells, usually containing no chloroplasts, with a thick outer wall covered by a heavy waxy cuticle about 2 pm thick. Only a few specialized cells are found in the epidermis. Among them are the paired guard cells that surround the small openings known as stomata on the undersurfaces of leaves and control transpiration of water. Specialized cells in the root epidermis form root hairs, long extensions ( 1 mm) of diameter 5-17 pm. Each hair is a single cell with the nucleus located near the tip. 

An HPLC method using progressive electrochemical detection of SPA was described by McCabe and Acworth (128). Samples were mixed with hexane, and SPA were extracted with acetonitrile. An HPLC analysis of the extracts was performed, without an evaporation step, on a high-pressure Coul Array system in which analytes were detected on two coulometric array-cell modules, each containing four electrochemical sensors attached in series after the column. Analytes were separated on a Supelcosil LC-18, 5-/tm column using gradient elution and detected at potentials of —50, 0, 70, 250, 375, 500, 675, and 825 mV. To remove oxidative impurities to be coeluted with BHT, a guard cell with applied potential of 900 mV was also placed in the system. 

In some plants, high PEPCase activities have been detected in specialised tissues or cells, other than the mesophyll cells of C4 and CAM plants. For example, PEPCase is found in guard cells where it is involved in stomatal opening and closing (Tarczynski Outlaw, 1990). The authors concluded, however, that - based on the alterations in chemical environment during stomatal activity - the reactions catalysed could be accomplished by a C3-type PEPCase enzyme and that there was no reason to invoke the existence of isoforms. Less well known is the PEPCase activity and protein detected in reproductive tissues. For example, high PEPCase activity has been observed in seed pods of chickpeas (Singh,... 

Tarczynski, M.C. Outlaw, W.H. (1990). Partial characterization of guard-cell phosphoeno/pyruvate carboxylase kinetic datum collection in real time from single-cell activities. Archives of Biochemistry and Biophysics 280, 153-8. 

Homberg, C. Weiler, E.W. (1984). High-affinity binding sites for abscisic acid on the plasmalemma of Vida faba guard cells. Nature 310, 321-4. 

One further problem is the large overshoot in ABA production in wilted leaves. With applied ABA a doubling of the ABA content of the leaf is usually adequate for stomatal closure, while increases up to 40-fold have been reported in wilted leaves. However, extractions of whole leaves do not take into account the location of ABA within the leaf. Perhaps much of the hormone is sequestered in a compartment that has no access to the guard cells. Thus, it would be of much importance to determine the distribution of ABA at the tissue level as well as its intracellular location. Since ABA is a small water-soluble molecule, conventional fractionation techniques may not be suitable to determine its distribution in various organelles. A highly specific immunological method for detection of ABA has recently been developed (38, 39). It is conceivable that this technique could be further developed for determining the cellular localization of ABA as has already been done for the photoreceptor phytochrome (77, 78). 

Ever since the discovery, by Ledbetter and Porter,88 of microtubules below the surface of the plasma membrane, suggestions have been made that these structures play some role in microfibril orientation. The suggestion arose because of two observations that (I) the orientation of microtubules has very frequently, but not always, been observed to be parallel to the orientation of the microfibrils most recently synthesized, and (2) agents, such as colchicine, that disrupt microtubules interfere with the orientation, but not the synthesis, of cellulose microfibrils. The literature pertaining to these studies has been well reviewed by Robinson,4 Schnepf and coworkers,89 Hepler and Palevitz,90 and Heath.91 In sum, the present evidence seems to favor some role for microtubules in orientation in some cases, such as the studies on guard cells by Palevitz and Hepler,92 and a series of papers on Oocystis by Robinson and coworkers,84,93-95 the case for micro-... 

We use the term myrosin cell structurally (see above), and not functionally (i.e., if a cell contains myrosinase it should be called a myrosin cell). This structural definition should be used because myrosin cell is a well-established anatomical term for this special idioblast. We do not find it useful to coin a specific term for a cell just because a certain protein is expressed in it, especially now in the post-genomic era, where possibly thousands of different proteins can be found in every cell. Husebye et al. named guard cells in Arabidopsis myrosin guard cells because myrosinase was found in these cells.47 This is problematic since these cells do not... 

HUSEBYE, H., CHADCHAWAN, S., WINGE, P., THANGSTAD, O.P., BONES, A.M., Guard cell- and phloem idioblast-specific expression of thioglucoside glucohydrolase 1 (myrosinase) in Arabidopsis., Plant Physiol., 2002,128,1180-1188. 

---